In looking at what are the shortcomings of the film Prince Caspian, we would do well to look at what the author thought of such a project. C.S. Lewis himself was very much against film versions being made of his Chronicles of Narnia as he thought that it would lead to "buffoonery or nightmare". In many respects he was right: as children's tales they are very effective, but when as it were brought to a more realistic view film view they fall apart.
When placed in a realist setting the talking animals of Narnia are completely unconvincing, and this came apparent in the first filming of the Lion, Witch and the Wardrobe. It was a considerable improvement on the BBC television dramatisation which ended up using soft toys! Nevertheless despite the best efforts made the talking animals betray themselves as computer graphics.
Another example that could be said to be unrealistic is the idea of children sword fighting adults: it sounds attractive in a book but comes out as absurd in reality. Also the geography and features of Narnia are not at all convincing when placed in a realist setting.
Probably the most effective way to film the Chronicles of Narnia would be to do them in animated form, rather than in live action. A fairly presentable attempt at the Lion, Witch and the Wardrobe was made in the late 1970's by the Children's Television Workshop, which I remember watching in my Grandmother's bedroom as a small boy..
However, the recent version of Prince Caspian lets itself down in other ways that could have been avoided: in particular the liberties taken with the original story. I will describe them in another post.